Interesting observation 280x, I had to raise GPU vdc in bios to get stable above 18.4MHs (now 1.118v) ... (was stable at 1.087v using previous kernel).  Best for me 1160gpu/1500mem threads 1, TC 24576 (it does make a difference).  Temps rise dramatically with minor increase in voltage sadly and have capped my overclock attempts as over 75c is enough for me  I have been unable to lower mem clock it seems to be locked (powercolor R9 280x).  Cooler ambient temps coming but will not gain all that much as temps seem to rise exponentially with overclocking (nice heater LOL).
Congrats on profitibility St.Neman I am barely profitable with increased power requirement but the 1 DMD reward window will end soon enough ... take advantage while you can.  Got new (used) Mobo on the way and will soon get main miner back online (only 1 280x atm in entertainment box right now main miner mobo died in a puff of smoke LOL).
You should be able to push 73+ Mhs with 4 cards ... (volt mod and proper heatsink paste application).